Usually, when the internal temperature of an internal combustion engine is high during operation, an air-fuel mixture burns easily under the conditions of a relatively low fuel concentration and a high pressure. At the time of start-up of the engine, and when the internal temperature of the engine is high, the mixture is easier to burn at a relatively high fuel concentration.
Consequently, when the throttle valve is brought into a fully closed state rapidly from its fully open state when internal temperature of the engine is high, there will be a rapid increase in negative pressure due to intake in the interior of the engine, which may cause misfiring.
To cope with the above problem, there is known a secondary air supply system which is used as an accessory for an internal combustion engine equipped with a so-called secondary air injection type exhaust gas purifying apparatus, such as the one disclosed in Japanese Utility Model Publication No. 8725/71.
The conventional supply system operates according to temperatures of exhaust gas, whereby upon an increase in negative pressure due to the intake in a fully closed throttle valve at a high engine temperature, secondary air is supplied into an intake pipe to prevent misfiring. At a low engine temperature, the supply of secondary air into the intake pipe is stopped even when the throttle valve is fulley closed, so that an air-fuel mixture having a high fuel concentration is supplied into the engine, thereby permitting quick start-up.
However, in such a supply system, when the engine brake is applied and the throttle valve assumes its fully close state, causing the negative pressure in the intake pipe to drop to a large extent, secondary air is temporarily supplied into the intake pipe as mentioned above to form, a dilute mixture of a low fuel concentration. The output-generating, state of the engine is continued temporarily, thus causing a delayed response to the engine brake.
Control circuits for a vehicle include, for example, an ignition control circuit for an internal combustion circuit and an air supply control circuit for the same engine. In the former, the ignition timing is determined on the basis of detected signals on vehicular running conditions such as engine speed and intake pipe internal pressure. An example is disclosed in Japanese Published Patent Application No. 85177/87, which discloses a control circuit having solenoid valve which is opened or closed in accordance with signals representing vehicular running conditions, such as engine speed and intake pipe pressure, plus an outside air temperature to control the supply of air so that good running characteristics are obtainable.
According to one conventional construction, a single CPU is used to effect plural controls. However, in a control circuit concerned with setting of a carburetor as in the foregoing circuits, considerable time is required for the setting and modification is made to a large extent until completion of the setting. This requires much time and labor for the preparation of a program, making its completion difficult.